The present invention relates generally to semiconductor device processing equipment, and, more particularly, to a pressure assisted, electrostatic wafer holding apparatus and control method.
Chucks are used in various material processing systems to retain workpieces (e.g., semiconductor wafers, dielectric substrates, etc.) thereon in a mechanically stationary position while the system process the workpiece. In particular, semiconductor wafer chucks are used to hold substrates in place and/or provide a heat transfer medium to the substrate for various processing steps (e.g., chemical vapor deposition, sputtering, etching, etc.) implemented during the manufacture of semiconductor devices such as integrated circuits.
There are three general types of chuck mechanism implementations that may be found in existing industrial wafer processing tools: gravity-based chucks, mechanical chucks and electrostatic chucks. In a gravity based chuck, a wafer is free standing on the chuck and simply held in place by gravity. While this design is the simplest of the three, a significant drawback thereof is the fact that there is an uncontrollable amount of contact between the wafer and the chuck. This is particularly disadvantageous for processes requiring heat transfer between the wafer and the chuck, and especially in low-vacuum processing environments, as this results in a highly non-uniform temperature gradient on the wafer. In turn, this leads to poor thickness control (e.g., for CVD/ALD processes), as well as variations in film properties.
In a mechanical chuck, a wafer is mechanically clamped to the chuck by means of a retention device, such as an edge ring. While this approach works fairly well for wafers that are somewhat bowed, the use of a mechanical retaining device can generate contaminating particles, as well as create a shadowing effect around the edge of the wafer. In fact, mechanically based chucks are virtually non-existent for semiconductor tooling of 300 mm wafer sizes.
On the other hand, an electrostatic chuck (ESC) retains a wafer thereon by generating a charge differential between a surface of the wafer and one or more electrodes located within the body of the chuck. The ensuing electrostatic force developed between the wafer and the electrodes retains the wafer against the chuck body. The electrodes are typically insulated from the wafer by a relatively thin layer of dielectric material. There are several well-known techniques for generating the electrostatic force in an electrostatic chuck. In addition, a typical ESC has an array of raised bumps, the surfaces of which are coated with certain semi-conducting composites such that a high electric field may be established, but not so as to excessively overload the power supply. For example, when a DC voltage ranging from about 200 volts to about 750 volts is applied to the chuck, an electrostatic attraction between the wafer and the chuck is established. This will generally create good contact without the drawbacks associated with a mechanical chuck.
However, although electrostatic chucks are extensively used in many 200 mm and 300 mm tool sets, there are still some disadvantages associated with electrostatic chucking. For example, electrostatic chucking force is a function of temperature, as well as the backside material of the wafer. While a higher chucking force may be obtained at a higher chuck and wafer temperature, the raising of the wafer temperature may not be a viable option for certain processes if the raised temperature changes the nature and/or rate of the particular process. Furthermore, for wafers that are more severely bowed (particularly, for a compressively bowed wafer stack), there is insufficient initial contact between the backside of the wafer and the chuck to take advantage of the generated electrostatic forces for adequate retention.
Accordingly, it would be desirable to be able to implement a chucking apparatus and method that advantageously utilizes the advantages of both electrostatic and mechanical chucking, but that does not suffer from the drawbacks associated therewith.